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| ONE_AS_ONE | = | '1 AS one' |
Returns true if a record exists in the table that matches the
id or conditions given, or false otherwise. The argument can
take six forms:
-
Integer - Finds the record with this primary key.
-
String - Finds the record with a primary key corresponding to this string (such as
'5'). -
Array - Finds the record that matches these
find-style conditions (such as['name LIKE ?', "%#{query}%"]). -
Hash - Finds the record that matches these
find-style conditions (such as{name: 'David'}). -
false- Returns alwaysfalse. -
No args - Returns
falseif the table is empty,trueotherwise.
For more information about specifying conditions as a hash or array, see the Conditions section in the introduction to ActiveRecord::Base.
Note: You can't pass in a condition as a string (like name =
'Jamie'), since it would be sanitized and then queried
against the primary key column, like id = 'name =
\'Jamie\''.
Person.exists?(5)
Person.exists?('5')
Person.exists?(['name LIKE ?', "%#{query}%"])
Person.exists?(id: [1, 4, 8])
Person.exists?(name: 'David')
Person.exists?(false)
Person.exists?
# File activerecord/lib/active_record/relation/finder_methods.rb, line 310 def exists?(conditions = :none) if Base === conditions conditions = conditions.id ActiveSupport::Deprecation.warn(" You are passing an instance of ActiveRecord::Base to `exists?`. Please pass the id of the object by calling `.id`. ".squish) end return false if !conditions relation = apply_join_dependency(self, construct_join_dependency) return false if ActiveRecord::NullRelation === relation relation = relation.except(:select, :order).select(ONE_AS_ONE).limit(1) case conditions when Array, Hash relation = relation.where(conditions) else unless conditions == :none relation = relation.where(primary_key => conditions) end end connection.select_value(relation, "#{name} Exists", relation.bound_attributes) ? true : false end
Find the fifth record. If no order is defined it will order by primary key.
Person.fifth # returns the fifth object fetched by SELECT * FROM people
Person.offset(3).fifth # returns the fifth object from OFFSET 3 (which is OFFSET 7)
Person.where(["user_name = :u", { u: user_name }]).fifth
Same as fifth but raises ActiveRecord::RecordNotFound if no record is found.
Find by id - This can either be a specific id (1), a list of ids (1, 5, 6),
or an array of ids ([5, 6, 10]). If one or more records can not be found
for the requested ids, then RecordNotFound will be raised. If the
primary key is an integer, find by id coerces its arguments using
to_i.
Person.find(1) # returns the object for ID = 1
Person.find("1") # returns the object for ID = 1
Person.find("31-sarah") # returns the object for ID = 31
Person.find(1, 2, 6) # returns an array for objects with IDs in (1, 2, 6)
Person.find([7, 17]) # returns an array for objects with IDs in (7, 17)
Person.find([1]) # returns an array for the object with ID = 1
Person.where("administrator = 1").order("created_on DESC").find(1)
NOTE: The returned records may not be in the same order as the ids you provide since database rows are unordered. You'd need to provide an explicit ActiveRecord::QueryMethods#order option if you want the results are sorted.
Find with lock
Example for find with a lock: Imagine two concurrent transactions: each
will read person.visits == 2, add 1 to it, and save, resulting
in two saves of person.visits = 3. By locking the row, the
second transaction has to wait until the first is finished; we get the
expected person.visits == 4.
Person.transaction do
person = Person.lock(true).find(1)
person.visits += 1
person.save!
end
Variations of find
Person.where(name: 'Spartacus', rating: 4)
# returns a chainable list (which can be empty).
Person.find_by(name: 'Spartacus', rating: 4)
# returns the first item or nil.
Person.find_or_initialize_by(name: 'Spartacus', rating: 4)
# returns the first item or returns a new instance (requires you call .save to persist against the database).
Person.find_or_create_by(name: 'Spartacus', rating: 4)
# returns the first item or creates it and returns it.
Alternatives for find
Person.where(name: 'Spartacus', rating: 4).exists?(conditions = :none)
# returns a boolean indicating if any record with the given conditions exist.
Person.where(name: 'Spartacus', rating: 4).select("field1, field2, field3")
# returns a chainable list of instances with only the mentioned fields.
Person.where(name: 'Spartacus', rating: 4).ids
# returns an Array of ids.
Person.where(name: 'Spartacus', rating: 4).pluck(:field1, :field2)
# returns an Array of the required fields.
Finds the first record matching the specified conditions. There is no implied ordering so if order matters, you should specify it yourself.
If no record is found, returns nil.
Post.find_by name: 'Spartacus', rating: 4
Post.find_by "published_at < ?", 2.weeks.ago
Like find_by, except that if no record is found, raises an ActiveRecord::RecordNotFound error.
Find the first record (or first N records if a parameter is supplied). If no order is defined it will order by primary key.
Person.first # returns the first object fetched by SELECT * FROM people ORDER BY people.id LIMIT 1
Person.where(["user_name = ?", user_name]).first
Person.where(["user_name = :u", { u: user_name }]).first
Person.order("created_on DESC").offset(5).first
Person.first(3) # returns the first three objects fetched by SELECT * FROM people ORDER BY people.id LIMIT 3
Same as first but raises ActiveRecord::RecordNotFound if no record is found. Note that first! accepts no arguments.
Find the forty-second record. Also known as accessing “the reddit”. If no order is defined it will order by primary key.
Person.forty_two # returns the forty-second object fetched by SELECT * FROM people
Person.offset(3).forty_two # returns the forty-second object from OFFSET 3 (which is OFFSET 44)
Person.where(["user_name = :u", { u: user_name }]).forty_two
Same as forty_two but raises ActiveRecord::RecordNotFound if no record is found.
Find the fourth record. If no order is defined it will order by primary key.
Person.fourth # returns the fourth object fetched by SELECT * FROM people
Person.offset(3).fourth # returns the fourth object from OFFSET 3 (which is OFFSET 6)
Person.where(["user_name = :u", { u: user_name }]).fourth
Same as fourth but raises ActiveRecord::RecordNotFound if no record is found.
Find the last record (or last N records if a parameter is supplied). If no order is defined it will order by primary key.
Person.last # returns the last object fetched by SELECT * FROM people
Person.where(["user_name = ?", user_name]).last
Person.order("created_on DESC").offset(5).last
Person.last(3) # returns the last three objects fetched by SELECT * FROM people.
Take note that in that last case, the results are sorted in ascending order:
[#<Person id:2>, #<Person id:3>, #<Person id:4>]and not:
[#<Person id:4>, #<Person id:3>, #<Person id:2>]# File activerecord/lib/active_record/relation/finder_methods.rb, line 147 def last(limit = nil) return find_last(limit) if loaded? || limit_value result = limit(limit || 1) result.order!(arel_attribute(primary_key)) if order_values.empty? && primary_key result = result.reverse_order! limit ? result.reverse : result.first rescue ActiveRecord::IrreversibleOrderError ActiveSupport::Deprecation.warn(" Finding a last element by loading the relation when SQL ORDER can not be reversed is deprecated. Rails 5.1 will raise ActiveRecord::IrreversibleOrderError in this case. Please call `to_a.last` if you still want to load the relation. ".squish) find_last(limit) end
Same as last but raises ActiveRecord::RecordNotFound if no record is found. Note that last! accepts no arguments.
Find the second record. If no order is defined it will order by primary key.
Person.second # returns the second object fetched by SELECT * FROM people
Person.offset(3).second # returns the second object from OFFSET 3 (which is OFFSET 4)
Person.where(["user_name = :u", { u: user_name }]).second
Same as second but raises ActiveRecord::RecordNotFound if no record is found.
Find the second-to-last record. If no order is defined it will order by primary key.
Person.second_to_last # returns the second-to-last object fetched by SELECT * FROM people
Person.offset(3).second_to_last # returns the second-to-last object from OFFSET 3
Person.where(["user_name = :u", { u: user_name }]).second_to_last
Same as second_to_last but raises ActiveRecord::RecordNotFound if no record is found.
Gives a record (or N records if a parameter is supplied) without any implied order. The order will depend on the database implementation. If an order is supplied it will be respected.
Person.take # returns an object fetched by SELECT * FROM people LIMIT 1
Person.take(5) # returns 5 objects fetched by SELECT * FROM people LIMIT 5
Person.where(["name LIKE '%?'", name]).take
Same as take but raises ActiveRecord::RecordNotFound if no record is found. Note that take! accepts no arguments.
Find the third record. If no order is defined it will order by primary key.
Person.third # returns the third object fetched by SELECT * FROM people
Person.offset(3).third # returns the third object from OFFSET 3 (which is OFFSET 5)
Person.where(["user_name = :u", { u: user_name }]).third
Same as third but raises ActiveRecord::RecordNotFound if no record is found.
Find the third-to-last record. If no order is defined it will order by primary key.
Person.third_to_last # returns the third-to-last object fetched by SELECT * FROM people
Person.offset(3).third_to_last # returns the third-to-last object from OFFSET 3
Person.where(["user_name = :u", { u: user_name }]).third_to_last
Same as third_to_last but raises ActiveRecord::RecordNotFound if no record is found.
# File activerecord/lib/active_record/relation/finder_methods.rb, line 529 def find_nth(index, offset = nil) # TODO: once the offset argument is removed we rely on offset_index # within find_nth_with_limit, rather than pass it in via # find_nth_with_limit_and_offset if offset ActiveSupport::Deprecation.warn(" Passing an offset argument to find_nth is deprecated, please use Relation#offset instead. ".squish) end if loaded? @records[index] else offset ||= offset_index @offsets[offset + index] ||= find_nth_with_limit_and_offset(index, 1, offset: offset).first end end
# File activerecord/lib/active_record/relation/finder_methods.rb, line 564 def find_nth_from_last(index) if loaded? @records[-index] else relation = if order_values.empty? && primary_key order(arel_attribute(primary_key).asc) else self end relation.to_a[-index] # TODO: can be made more performant on large result sets by # for instance, last(index)[-index] (which would require # refactoring the last(n) finder method to make test suite pass), # or by using a combination of reverse_order, limit, and offset, # e.g., reverse_order.offset(index-1).first end end
# File activerecord/lib/active_record/relation/finder_methods.rb, line 551 def find_nth_with_limit(index, limit) # TODO: once the offset argument is removed from find_nth, # find_nth_with_limit_and_offset can be merged into this method relation = if order_values.empty? && primary_key order(arel_attribute(primary_key).asc) else self end relation = relation.offset(index) unless index.zero? relation.limit(limit).to_a end
# File activerecord/lib/active_record/relation/finder_methods.rb, line 465 def find_one(id) if ActiveRecord::Base === id id = id.id ActiveSupport::Deprecation.warn(" You are passing an instance of ActiveRecord::Base to `find`. Please pass the id of the object by calling `.id`. ".squish) end relation = where(primary_key => id) record = relation.take raise_record_not_found_exception!(id, 0, 1) unless record record end
# File activerecord/lib/active_record/relation/finder_methods.rb, line 482 def find_some(ids) return find_some_ordered(ids) unless order_values.present? result = where(primary_key => ids).to_a expected_size = if limit_value && ids.size > limit_value limit_value else ids.size end # 11 ids with limit 3, offset 9 should give 2 results. if offset_value && (ids.size - offset_value < expected_size) expected_size = ids.size - offset_value end if result.size == expected_size result else raise_record_not_found_exception!(ids, result.size, expected_size) end end
# File activerecord/lib/active_record/relation/finder_methods.rb, line 506 def find_some_ordered(ids) ids = ids.slice(offset_value || 0, limit_value || ids.size) || [] result = except(:limit, :offset).where(primary_key => ids).records if result.size == ids.size pk_type = @klass.type_for_attribute(primary_key) records_by_id = result.index_by(&:id) ids.map { |id| records_by_id.fetch(pk_type.cast(id)) } else raise_record_not_found_exception!(ids, result.size, ids.size) end end
# File activerecord/lib/active_record/relation/finder_methods.rb, line 444 def find_with_ids(*ids) raise UnknownPrimaryKey.new(@klass) if primary_key.nil? expects_array = ids.first.kind_of?(Array) return ids.first if expects_array && ids.first.empty? ids = ids.flatten.compact.uniq case ids.size when 0 raise RecordNotFound, "Couldn't find #{@klass.name} without an ID" when 1 result = find_one(ids.first) expects_array ? [ result ] : result else find_some(ids) end rescue RangeError raise RecordNotFound, "Couldn't find #{@klass.name} with an out of range ID" end